In some radiofrequency information transmission applications, it has been observed that the transmission or receiver antenna could have an impedance that was highly dependent on conditions external to the antenna, and notably dependent on the environment in which the antenna is placed.
For example, in medical telemetry, it may be necessary to introduce the antenna into a probe placed in the human body, and the impedance is then highly dependent on the biological environment in which the antenna is located. It depends on the electrical properties (conductivity, dielectric constant) of the surrounding tissues (muscles, fat) or of the liquid medium (blood, other liquids) into which the antenna may be immersed.
Even in more conventional applications of radiofrequency transmission (mobile telephony, etc.) the impedance of the antenna can vary.
A transmission (respectively receiver) channel comprises at least one amplifier with which one or more filters may be associated.
Generally speaking, the variations in antenna impedance are particularly sensitive for miniaturized antennas having a high quality factor, used in applications with severe size restrictions.
These variations in impedance can lead to losses known as ‘mismatch losses’: these losses result from the fact that the transmission channel which feeds the antenna, or the receiver channel which receives a signal from the antenna, is in general designed to have optimum performance when it is loaded (at the output for the transmission channel or at the input for the receiver channel) by a well-defined nominal impedance; it has degraded performance characteristics when it is loaded by an impedance different from its nominal value. The mismatch losses can be as high as 40 dB.
For this reason, attempts have been made to interpose between the output of the transmission channel and the transmission antenna (and it could also be done at the input for a receiver antenna) an impedance matching network, which means that the transmission channel sees an impedance different from that of the antenna and preferably equal to the nominal value for which it has been designed, for example 100 ohms or 500 ohms. The matching network is tunable, in other words its capacitive and/or inductive elements have adjustable values in order to take into account the environmental conditions of the antenna such that the matching is the best possible whatever the circumstances.
In the prior art illustrated by the U.S. Pat. No. 4,375,051, a bidirectional coupler is used to detect a mismatch: the power of the amplifier is applied to a load via the coupler and an impedance matching network; if the system is mismatched, a part of the power sent to the antenna is reflected by the latter instead of being emitted into the surrounding environment; the reflected part returns into the coupler and emerges from it via a specific output; the reflected output power is detected, measured, and is used as a feedback control to the impedance network to modify its characteristics in a direction tending to reduce the reflected power. The matching network comprises variable capacitors. The control of the network can be achieved by a feedback control loop based on hardware circuits or based on a calculation algorithm.
This process is iterative, slow, and power-consuming. The use of a coupler renders the matching circuit impossible to integrate into an integrated circuit chip due to the bulkiness of the coupler.
In the patent application US 2009-0066440, a method is furthermore provided for automatic impedance matching in a transmission or receiver channel, in which the amplitude and the phase of the current and of the voltage at the output of the transmission channel (or at the input of the receiver channel) are simultaneously detected. The ratio between the voltage and the current is representative of the load impedance Zm seen by the channel loaded by the combination of the matching network and of the antenna of impedance Zant. The load impedance Zm is measured and the antenna impedance Zant is calculated from the measured load impedance Zm and from the impedances of the matching network whose configuration is known at the time of the measurement, and finally, the modification that needs to be applied to one or more of the impedances of the matching network is calculated, in order to obtain the result that the impedance seen by the amplifier becomes matched to the nominal impedance of the amplifier in the current environmental conditions of the antenna.
However, the circuit described in this patent application is designed to operate at a well-defined radiofrequency and it does not allow for correct operation if the operating frequency of the channel is modified.